firsts
FIRSTS list
outputs a list containing the FIRST of each member of the input
list. It is an error if any member of the input list is empty.
(The input itself may be empty, in which case the output is also
empty.) This could be written as
to firsts :list
output map "first :list
end
but is provided as a primitive in order to speed up the iteration
tools MAP, MAP.SE, and FOREACH.